Liquid crystal display devices are thin, lightweight, and low power consuming. Owning to these and other characteristics, the liquid crystal display device has been widely used for a wide variety of devices as large as television sets and personal computers and as small as cameras, videos, portable phones, and portable terminals.
The liquid crystal display device is equipped with a liquid crystal panel. In a liquid crystal panel using polycrystalline (poly) silicon (p-Si) transistors as pixel transistors for driving pixels, peripheral driving circuits formed of p-Si TFT can be integrated in non-display areas.
The driving circuits of such liquid crystal display devices include components that preferably require fast operations, such as display controllers, shift registers, or the like, and components that preferably require high voltage resistance, such as output buffers, level shifters, analog switches, or the like. TFTs that require fast operation preferably have a short channel length and no LDD structure. On the other hand, TFTs that require high voltage resistance need to be voltage resistant rather than having fast operation. Pixel TFTs require high voltage resistance rather than fast operation. In order to withstand a required level of high voltage, such voltage-resistant TFTs preferably have a TFT structure with a gate insulating film of a sufficient thickness, and with a LDD (lightly doped drain, low-density region).
The pixel driving TFT preferably operates so that it writes data voltage when the gate voltage is ON and holds the data voltage until the next data voltage is written in. It is therefore preferable that the leak current be reduced as much as possible while the gate voltage is OFF. The leak current can be preferably reduced by providing a low-density region (lightly doped drain, LDD) between the channel of p-Si TFT and a low-resistant (high-density) source-drain region. For this reason, the pixel transistor is formed of n-channel TFT (NTFT), which performs better than p-channel TFT (PTFT).
The peripheral circuits include circuits such as high voltage-resistant input/output circuits and logic circuits. The logic circuit such as a shift register preferably requires fast operation. It is therefore preferable that the p-Si TFTs for the peripheral circuits be provided as high-voltage resistant transistors and fast-operating transistors. As such, the peripheral circuits are formed of complementary MOS (CMOS) TFTs using NTFT and PTFT.
In the following, description is made as to conventional techniques relating to TFT.
As described in the PRIOR ART section and other parts of Japanese Laid-Open Patent Publication No. 2000-299469 (published on Oct. 24, 2000), OFF current of p-Si TFT tends to increase, whereas ON current tends to decrease by the hot carrier effect. It is believed that the decrease of ON current occurs as the hot carrier generated by the strong electric field in the vicinity of the drain is trapped in a gate insulating film. In a GOLD (gate-drain overlapped LDD) structure in which the gate electrodes overlap the LDD region, the strong electric field in the vicinity of the drain is relieved and the hot carrier effect is attenuated. In order to provide the LDD region below the gate electrodes, the LDD region needs to be formed before forming the gate electrodes, using a mask.
The liquid crystal panel is formed using an insulating substrate such as glass. Because the substrate is insulating, the TFTs formed thereon are susceptible to destruction by static electricity. The substrate is protected from electrostatic discharge (ESD) by forming a wiring pattern called a “short ring,” which shorts the ends of a date line (drain bus line) and a scan line (gate bus line), and which is removed before the fabrication of the liquid crystal panel completes. In a liquid crystal panel in which the peripheral circuits are integrated on a substrate, it is desirable to perform operation tests for the peripheral circuits. For this purpose, terminals to be used for the operation tests are drawn out from the peripheral circuits.
Japanese Laid-Open Patent Publication No. 202289/1999 (Tokukaihei 11-202289, published on Jul. 30, 1999) discloses a liquid crystal display device that allows for element evaluation without cutting the short line (short ring), and is capable of performing display without cutting the short line after the liquid crystal display device is fabricated. The influence of static electricity is also reduced in this liquid crystal display device. The foregoing publication realizes such a liquid crystal display device by proposing connecting the gate bus line and drain bus line to the short line via a connecting TFT, applying a high voltage across a gate and a source to electrically shift a threshold value, and turning on the connecting TFT during the fabrication process and turning off the connecting TFT during characteristics evaluation and after the fabrication. In an embodiment of this publication, amorphous silicon TFT is used and the peripheral circuits are not integrated.
Japanese Laid-Open Patent Publication No. 68110/1999 (Tokukaihei 11-68110, published on Mar. 9, 1999) discloses forming a plurality of aluminum short rings surrounding peripheral circuits and each active-matrix circuit provided on a single glass substrate, connecting the short rings with a Si thin film, and injecting impurities to the Si thin film when forming impurity regions in the TFTs of the active-matrix circuits and peripheral circuits.
Japanese Laid-Open Patent Publication No. 10116/2000 (Tokukai 2000-10116, published on Jan. 14, 2000) discloses an arrangement in which drain bus lines, gate bus lines, and peripheral circuit terminals for operation tests are drawn out to a non-display area, and are connected to a terminal wiring (short ring) using a resistance wiring made of polysilicon, as shown in FIG. 6 and described in conjunction with FIG. 6 for example. In the liquid crystal display device disclosed in this publication, the resistance of the resistance wiring is suitably selected, enabling testing to be properly carried out even without cutting the terminal wiring. The peripheral circuit and the short ring are disconnected from each other at the polysilicon resistance wiring, before fabrication of the panel is completed.